Same thing's happened with Broadchurch Series 1. We only discovered Broadchurch at Series 3 so when ITV Boxsets went HD a month or so back we bookmarked the first two series for saving. Series 2 still there but 1's gone. Oh well, plenty of other stuff knocking around.
At a guess I'd say all of ITV VOD stuff has been refreshed today. I just wish there was some way of checking expiry dates of these things, there used to be a hidden webpage somewhere but I never got on with that.